The material properties of secondary-hardened carbon stainless steels are often limited by cementite precipitation during aging. Because the cementite is enriched with alloying elements, it becomes more difficult to fully dissolve the cementite as the alloying content of elements such as chromium increases. Undissolved cementite in the steel can limit toughness, reduce strength by gettering carbon, and act as corrosion pitting sites.
Cementite precipitation could be substantially suppressed in stainless steels by substituting nitrogen for carbon. There are generally two ways of using nitrogen in stainless steels for strengthening: (1) solution-strengthening followed by cold work; or (2) precipitation strengthening. Cold worked alloys are not generally available in heavy cross-sections and are also not suitable for components requiring intricate machining. Therefore, precipitation strengthening is often preferred to cold work. Precipitation strengthening is typically most effective when two criteria are met: (1) a large solubility temperature gradient in order to precipitate significant phase fraction during lower-temperature aging after a higher-temperature solution treatment, and (2) a fine-scale dispersion achieved by precipitates with lattice coherency to the matrix.
These two criteria are difficult to meet in conventional nitride-strengthened martensitic steels. The solubility of nitrogen is very low in the high-temperature bcc-ferrite matrix, and in austenitic steels, nitrides such as M2N are not coherent with the fcc matrix. Thus, there has developed a need for a martensitic steel strengthened by nitride precipitates.
Stainless steel alloys are commonly used in structural applications demanding high strength, ductility and corrosion resistance. Specifically, high-performance, stainless bearing steel is needed to achieve long life and efficient operation of aerospace drive system turbine machinery operating in a corrosive environment. For example, vertical take-off and landing lift-systems in modern jet turbine engines have gears and bearings that are often subject to moist air. Compared to most gearbox assemblies, these lift-system gearbox assemblies are not in service long enough to ensure all of the moisture is driven off during operation due to heat. As a result, condensation results in corrosion, especially on carburized surfaces. Available aerospace gear alloys such as 440C, AMS 6308, 9310 (AMS 6256), FERRIUM® C61 (AMS 6517), and FERRIUM® C64 (AMS 6509) have limited corrosion resistance. Other options may also provide some level of corrosion resistance, such as in PYROWEAR® 675 (AMS 5930), but corrosion resistance is compromised due to a suboptimal case carburized microstructure and low matrix chromium content. It would be advantageous to develop a fully stainless, surface hardenable steel alloy alternative with improved corrosion resistance and enhanced bearing performance.